Liquid crystal display devices are widely used in OA (office automation) equipment, car televisions, monitors for camcorders, etc., for their advantageous features such as light weight, thin structure and small power consumption. Unlike self-luminous display devices such as CRTs, PDPs (plasma display panels) and EL (electroluminescence) devices, liquid crystal display devices use a liquid crystal display element that itself does not produce light. Therefore, in a transmission type liquid crystal display device, a planar illuminator called a “backlight” is provided on the back side of the liquid crystal display element, and the liquid crystal display element controls the amount of output light from the backlight to be transmitted therethrough in each pixel so as to display an image.
Backlights are generally classified into “direct-type” backlights including a plurality of rod-shaped light sources, such as fluorescent tubes, placed directly under a liquid crystal display element, and “edge light-type” backlights including a light source placed along an edge of a lightguide plate so that light from the light source is guided through the lightguide plate to a liquid crystal display element.
A typical structure of a direct-type backlight is illustrated in FIG. 21. A backlight 40 illustrated in FIG. 21 is provided on the back side of a transmission type liquid crystal display panel 48, and includes a plurality of light sources (fluorescent tubes) 41 arranged at regular intervals, a case 42 accommodating the light sources 41, and a diffusion plate 43 provided between the light sources 41 and the liquid crystal display panel 48. The diffusion plate 43 diffuses light output from the light source 41 to increase the uniformity of the output light. The diffusion plate 43 is typically made of a material including a resin and particles dispersed in the resin and having a different refractive index from that of the resin. Because of the difference between the refractive index of the resin matrix and that of the particles dispersed therein, a light-diffusing property is exhibited. An acrylic resin is often used as the resin because it is highly transparent and is easy to mold, and silica beads are often used as the particles because they are highly transparent.
In recent years, since a liquid crystal display device is required to have a very high brightness, the number of the light sources 41 used is increasing, and the amount of heat generated from the light sources 41 is also increasing. However, the diffusion plate 43 is made of a material containing a resin with a low thermal conductivity (for example, acrylic resin has a very low thermal conductivity), and has a low heat-radiating property. Therefore, if a large number of light sources 41 are provided, heat radiation of the diffusion plate 43 may be insufficient, and the temperature distribution across the diffusion plate 43 may become non-uniform, whereby the temperature distribution across the display plane of the liquid crystal display panel 48 may also become non-uniform. Since the optical and electrical characteristics of a liquid crystal material sealed in the liquid crystal display panel 48 are temperature-dependent, a non-uniform temperature distribution across the liquid crystal display panel 48 causes a brightness non-uniformity, and the like, thereby lowering the display quality. Moreover, an excessively high temperature inside the backlight 40 leads to other problems such as a decrease in the luminous efficiency of the light sources 41 and softening of resin-made components. Moreover, as the thickness of a backlight is decreasing recently, the distance between the light source 41 and the diffusion plate 43 is decreasing, whereby such problems are more likely to occur.
In view of this, Japanese Laid-Open Patent Publication No. 4-172319 discloses a method in which a diffusion plate of a backlight is made of a resin material mixed with minute glass particles, thereby improving the heat-radiating property of the diffusion plate.
However, in-depth research by the present inventor revealed that a heat-radiating property that is sufficient in practice cannot be obtained by forming a diffusion plate using a resin material mixed with minute glass particles as disclosed in Japanese Laid-Open Patent Publication No. 4-172319.
These problems occur not only with direct-type backlights but also with edge light-type backlights due to the heat-radiating property of the lightguide plate (lightguide) being insufficient.